The present invention relates to an automatic mold thickness adjusting method for a toggle-type mold clamping apparatus of, e.g., an injection-molding machine, and more particularly, to an automatic mold thickness adjusting method capable of speedy mold thickness adjustment using a low-priced mold clamping apparatus.
A toggle-type mold clamping apparatus attached to an injection-molding machine typically comprises a toggle mechanism, which includes toggle links formed of link members connected individually to a rear platen and a movable platen and also to each other. The toggle mechanism is driven by a motor to establish a lockup state in which the toggle links stretch to their full length, whereby tie bars, opposite ends of which are connected individually to the front stationary platen and the rear platen, are stretched to produce a mold clamping force. In replacing a die, moreover, a motor for mold thickness adjustment is driven to adjust automatically the position of the rear platen, i.e., to effect a mold thickness adjustment in accordance with the thickness of the die and a required mold clamping force.
Conventionally, mold clamping apparatuses are known which use servomotors for both the motor for the toggle mechanism and the motor for mold thickness adjustment (refer to Japanese Patent Disclosure Nos. 61-71164, 61-220819, and 61-249729). Using the expensive servomotors, however, the apparatuses of this type entail high costs.
Accordingly, there is a proposal that the costs of a mold clamping apparatus be lowered by using a general-purpose motor, such as an induction motor, as the motor for mold thickness adjustment (refer to Japanese Patent Disclosure No. 62-220314). In the mold clamping apparatus according to this proposal, if a die-touch state, in which one die half attached to a movable platen engages the other die half attached to a front stationary platen, is attained before toggle links stretch to their full length, during the drive of a motor for a toggle mechanism for mold thickness adjustment, the motor for mold thickness adjustment is driven to cause a rear platen to retreat for a fixed distance after the motor for the toggle mechanism is stopped, and the motor for the toggle mechanism is then driven to cause the movable platen to advance for the fixed distance. The retreat of the rear platen and the advance of the movable platen are repeatedly executed until the toggle links stretch to their full length. Thus, the two motors are alternately driven to effect the mold thickness adjustment, so that the proposed apparatus requires a long time for the mold thickness adjustment.
If the toggle links stretch to their full length before the die-touch state is attained, during the mold thickness adjustment, the motor for mold thickness adjustment is driven to advance the rear platen, thereby causing the dies to engage each other, and the same motor is further driven to advance the rear platen further, while causing the toggle links to bend. Then, after the rear platen is retreated for a predetermined period of time, the toggle links are caused to stretch again to their full length. With the toggle links stretched to their full length, the rear platen is advanced for a period of time longer than the predetermined period of time, whereupon the mold thickness adjustment is finished. Thus, in the proposed apparatus described above, the motor for mold thickness adjustment is driven in the die-touch state, during the mold thickness adjustment, so that an overcurrent undesirably flows through this motor.